In treating disease caused by proliferative tissue disorders such as cancer and coronary artery restenosis with radiation, the portions of the patient known to contain or suspected to contain disease are irradiated. For this purpose, a radiotherapy planning system is used to first acquire planning images of the diseased portion(s) and surrounding regions.
Radiotherapy planning systems generally include a CT or magnetic resonance imaging (MRI) simulator. CT or MRI radiography is carried out on a single day before the beginning of therapy to acquire a plurality of coregistered sectional 2-D images. These sectional images are combined using known algorithms to produce 3-D images. These 3-D simulation images are displayed and then analyzed to identify the location of regions of suspected disease to be treated, such as a radiographically evident tumor or regions suspected of microscopic disease spread. These regions to be treated are called radiotherapy targets. In order to attempt to account for organ motions, the concept of margins and planning target volumes (PTVs) was developed to attempt to irradiate a volume that would hopefully contain the target during most of the irradiation. PTVs include a geometric margin to account for variations in patient geometry or motion. Likewise, the 3-D simulation images are displayed and then analyzed to identify important normal anatomy and tissues that may be damaged by the radiation, such as the spinal cord and lung, to evaluate the potential impact of radiation on the function of these tissues. These regions to be spared or protected from excessive radiation are called critical structures or organs at risk and may also include a margin to account for variations in patient geometry or motion. The delivery of radiation therapy is then traditionally planned on a single static model of radiotherapy targets and critical structures derived from a single set of CT and/or MRI images. Because the known art does not allow for simultaneous imaging and therapy, the patient and all of their internal organs need to be repositioned exactly for accurate dose delivery. However, it is known in the art that exactly repositioning the patient even for a single delivery of dose is not possible due to several factors including: the inability to reproduce the patient setup, i.e., the geometry and alignment of the patient's body; physiological changes in the patient, such as weight loss or tumor growth and shrinkage; and organ motions in the patients including but not limited to breathing motion, cardiac motion, rectal distension, peristalsis, bladder filling, and voluntary muscular motion. Note that the organ motions may occur on rapid time scales such that changes may occur during a single dose delivery (e.g., breathing motion), termed “intra-fraction” organ motions, or they may occur on slower time scales such that changes occur in between dose deliveries, termed “inter-fraction” organ motions. Much of the curative treatment of patients with cancer outside the cranium requires the delivered radiation therapy to be fractionated, i.e., the dose is delivered in many fractions. Typically, dose is delivered in single 1.8 to 2.2 Gy fractions or double 1.2 to 1.5 Gy fractions daily, and delivered during the work week (Monday through Friday); taking 7 to 8 weeks to deliver, e.g., a cumulative dose of 70 to 72 Gy at 2.0 or 1.8 Gy, respectively. A purpose of this invention is to overcome the limitations imposed on radiation therapy by patient setup errors, physiological changes, and both intra- and inter-fraction organ motions throughout the many weeks of radiation therapy. Another purpose is to allow the physician to periodically monitor the response of the patient's disease to the therapy by performing MRI that provides metabolic and physiological information or assessing the growth or shrinkage of gross disease.
An irradiation field shape is then determined to coincide with an outline of an image of the target's diseased regions or suspected regions appearing in the planning images. An irradiating angle is determined from sectional images of a wide region including the diseased portion or a transmitted image, seen from a particular direction, produced by the 3-D simulation images. A transmitted image seen from the irradiating angle is displayed. The operator then determines a shape of an irradiation field on the image displayed, and sets an isocenter (reference point) to the irradiation field.
Optionally, the patient may be positioned relative to a conventional simulator (ortho-voltage X-ray imaging system that allows portal images to be generated for radiation therapy setup). An irradiating angle corresponding to the irradiating angle determined as above is set to the simulator, and an image is generally radiographed on a film through radiography for use as a reference radiograph. Similar digitally reconstructed radiographs may be produced using CT or MRI simulation software.
The patient is then positioned and restrained relative to a radiation treating apparatus which generally includes a radiation source, typically a linear accelerator. An irradiating angle is set to the irradiating angle determined as above, and film radiography is carried out by emitting radiation from the radiation treating apparatus. This radiation film image is correlated with the above film image acting as the reference radiograph to confirm that the patient has been positioned according to plan as correctly as possible before proceeding with radiotherapy. Some repositioning is generally required to place the patient such that the structures in the reference radiograph match the structures in the treatment radiograph to within a tolerance of 0.2 to 0.5 cm. After acceptable patient positioning is confirmed, radiotherapy is begun.
Patient setup errors, physiological changes, and organ motions result in increasing misalignment of the treatment beams relative to the radiotherapy targets and critical structures of a patient as the radiotherapy process proceeds. For years practitioners have been acquiring hard-copy films of the patient using the radiation therapy beam, technically referred to as a “port film” to attempt to ensure that the beam position does not significantly vary from the original plan. However, the port films acquired are generally only single 2-D projection images taken at some predetermined interval during the radiotherapy process (typically 1 week). Port films cannot account for organ motion. Additionally, port films do not image soft tissue anatomy with any significant contrast and only provide reliable information on the bony anatomy of the patient. Accordingly, misalignment information is only provided at the instants in time in which the port images are taken and may be misleading as the bony anatomy and soft tissue anatomy alignment need not correlate and change with time. With appropriate markers in the port image provided, the beam misalignment may be determined and then corrected to some limited degree.
More recently, some have disclosed acquiring the port images electronically, referred to as electronic portal imaging. This imaging technique employs solid state semiconductor, scintillator, or liquid ionization chamber array technology to capture x-ray transmission radiographs of the patient using the x-rays of the linear accelerator or an associated kilovoltage x-ray unit. As with the hard-copy technique, misalignment data is only provided at the instants in time in which the port images are taken. Another recent advance in electronic portal imaging includes the use of implanted interstitial radio-opaque markers that attempt to image the location of soft tissues. These procedures are invasive and subject to marker migration. Even when performed with the rapid acquisition of many images, it only finds the motion of discrete points identified by the radio-opaque markers inside a soft tissue and cannot account for the true complexities of organ motions and the dosimetric errors that they cause. Another recent advance, that creates 3D volumetric image sets from many 2D electronic portal images, is the acquisition of volumetric cone-beam x-ray CT or helical tomotherapy megavoltage x-ray CT image set before or after the daily delivery of therapy. While this technology may account for patient setup errors, i.e., the geometry and alignment of the patient's body, physiological changes in the patient, such as weight loss or tumor growth and shrinkage, and inter-fraction organ motions in the patient, such as rectal filling and voiding; it cannot account for intra-fraction organ motions in the patients. Intrafraction organ motions are very important and include, but are not limited to, breathing motion, cardiac motion, rectal gas distension, peristalsis, bladder filling, and voluntary muscular motion.
Radiation therapy has historically been delivered to large regions of the body including the target volume. While some volume margin is required to account for the possibility of microscopic disease spread, much of the margin is required to account for uncertainties in treatment planning and delivery of radiation. Reducing the total volume of tissue irradiated is beneficial, since this reduces the amount of normal tissue irradiated and therefore reduces the overall toxicity to the patient from radiation therapy. Furthermore, reduction in overall treatment volume may allow dose escalation to the target, thus increasing the probability of tumor control.
Clinical cobalt (Co60 radioisotope source) therapy units and MV linear accelerators (or linacs) were introduced nearly contemporaneously in the early 1950's. The first two clinical cobalt therapy units were installed nearly simultaneously in October of 1951 in Saskatoon and London, Ontario. The first MV linear accelerator installed solely for clinical use was at Hammersmith Hospital, London England in June of 1952. The first patient was treated with this machine in August of 1953. These devices soon became widely employed in cancer therapy. The deeply penetrating ionizing photon beams quickly became the mainstay of radiation therapy, allowing the widespread noninvasive treatment of deep seated tumors. The role of X-ray therapy slowly changed with the advent of these devices from a mainly palliative therapy to a definitive curative therapy. Despite similarities, cobalt units and linacs were always viewed as rival technologies in external beam radiotherapy. This rivalry would result in the eventual dominance of linacs in the United States and Western Europe. The cobalt unit was quite simplistic and was not technically improved significantly over time. Of course, the simplicity of the cobalt unit was a cause for some of its appeal; the cobalt units were very reliable, precise, and required little maintenance and technical expertise to run. Early on, this allowed cobalt therapy to become the most widespread form of external beam therapy. The linac was the more technically intensive device. Accelerating high currents of electrons to energies between 4 and 25 MeV to produce beams of bremsstrahlung photons or scattered electrons, the linac was a much more versatile machine that allowed more penetrating beams with sharper penumbrae and higher dose rates. As the linac became more reliable, the benefits of having more penetrating photon beams coupled with the addition of electron beams was seen as strong enough impetus to replace the existing cobalt units. Cobalt therapy did not die away without some protests and the essence of this debate was captured in a famous paper in 1986 by Laughlin, Mohan, and Kutcher which explained the pros and cons of cobalt units with linacs. This was accompanied by an editorial from Suit that pleaded for the continuance and further technical development of cobalt units. The pros of cobalt units and linacs have already been listed. The cons of cobalt units were seen as less penetrating depth dose, larger penumbra due to source size, large surface doses for large fields due to lower energy contamination electrons, and mandatory regulatory oversight. The cons for linacs increased with their increasing energy (and hence their difference from a low energy cobalt beam), and were seen to be increased builddown, increased penumbra due to electron transport, increased dose to bone (due to increased dose due to pair production), and most importantly the production of photo-neutrons at acceleration potentials over 10 MV.
In the era before intensity modulated radiation therapy (IMRT), the linac held definite advantages over cobalt therapy. The fact that one could produce a very similar beam to cobalt using a 4 MV linac accelerating potential combined with the linac's ability to produce either electron beams or more penetrating photon beams made the linac preferable. When the value of cobalt therapy was being weighed against the value linac therapy, radiation fields were only manually developed and were without the benefit of IMRT. As IMRT has developed, the use of higher MV linac accelerating potential beams and electron beams have been largely abandoned by the community. This is partly due to the increased concern over neutron production (and increased patient whole body dose) for the increased beam-on times required by IMRT and the complexity of optimizing electron beams, but most importantly because low MV photon-beam IMRT could produce treatment plans of excellent quality for all sites of cancer treatment.
IMRT represents a culmination of decades of improving 3D dose calculations and optimization to the point that we have achieved a high degree of accuracy and precision for static objects. However, there is a fundamental flaw in our currently accepted paradigm for dose modeling. The problem lies with the fact that patients are essentially dynamic deformable objects that we cannot and will not perfectly reposition for fractioned radiotherapy. Even for one dose delivery, intra-fraction organ motion can cause significant errors. Despite this fact, the delivery of radiation therapy is traditionally planned on a static model of radiotherapy targets and critical structures. The real problem lies in the fact that outside of the cranium (i.e., excluding the treatment of CNS disease using Stereotactic radiotherapy) radiation therapy needs to be fractionated to be effective, i.e., it must be delivered in single 1.8 to 2.2 Gy fractions or double 1.2 to 1.5 Gy fractions daily, and is traditionally delivered during the work week (Monday through Friday); taking 7 to 8 weeks to deliver a curative dose of 70 to 72 Gy at 2.0 or 1.8 Gy, respectively. This daily fractionation requires the patient and all of their internal organs to be repositioned exactly for accurate dose delivery. This raises an extremely important question for radiation therapy: “Of what use is all of the elegant dose computation and optimization we have developed if the targets and critical structures move around during the actual therapy?” Recent critical reviews of organ motion studies have summarized the existing literature up to 2001 and have shown that the two most prevalent types of organ-motion: patient set-up errors and organ motions. While significant physiological changes in the patient do occur, e.g., significant tumor shrinkage in head-and-neck cancer is often observed clinically, they have not been well studied. Organ motion studies have been further subdivided into inter-fraction and intra-fraction organ motion, with the acknowledgement that the two cannot be explicitly separated, i.e., intra-fraction motions obviously confound the clean observation of inter-fraction motions. Data on inter-fraction motion of gynecological tumors, prostate, bladder, and rectum have been published, as well as data on the intra-fraction movement of the liver, diaphragm, kidneys, pancreas, lung tumors, and prostate. Many peer-reviewed publications, spanning the two decades prior to publication have demonstrated the fact that both inter- and intra-fraction organ motions may have a significant effect on radiation therapy dosimetry. This may be seen in the fact that displacements between 0.5 and 4.0 cm have been commonly observed in studies of less than 50 patients. The mean displacements for many observations of an organ motion may be small, but even an infrequent yet large displacement may significantly alter the biologically effective dose received by a patient, as it is well accepted that the correct dose per fraction must be maintained to effect tumor control. In a more focused review of intra-fraction organ motion recently published by Goitein (Seminar in Radiation Oncology 2004 January; 14(1):2-9), the importance of dealing with organ motion related dosimetry errors was concisely stated: “ . . . it is incontestable that unacceptably, or at least undesirably, large motions may occur in some patients . . . ” It was further explained by Goitein that the problem of organ motions has always been a concern in radiation therapy: “We have known that patients move and breathe and that their hearts beat and their intestines wriggle since radiation was first used in cancer therapy. In not-so-distant decades, our solution was simply to watch all that motion on the simulator's fluoroscope and then set the field edge wires wide enough that the target (never mind that we could not see it) stayed within the field.”
In an attempt to address the limitations imposed on radiation therapy by patient setup errors, physiological changes, and organ motion throughout the protracted weeks of radiation therapy, the prior art has been advanced to imaging systems capable of acquiring a volumetric CT “snap shot” before and after each delivery of radiation. This new combination of radiation therapy unit with radiology imaging equipment has been termed image-guided radiation therapy (IGRT), or preferably image guided IMRT (IGIMRT). The prior art has the potential for removing patient setup errors, slow physiological changes, and inter-fraction organ motions that occur over the extended course of radiation therapy. However, the prior art cannot account for intra-fraction organ motion which is a very significant form of organ motion. The prior art devices are only being used to shift the gross patient position. The prior art cannot capture intra-fraction organ motion and is limited by the speed at which helical or cone-beam CT imaging may be performed Secondly, but perhaps equally important, CT imaging adds to the ionizing radiation dose delivered to the patient. It is well known that the incidence of secondary carcinogenesis occurs in regions of low-to-moderate dose and the whole body dose will be increased by the application of many CT image studies.
CT imaging and MRI units were both demonstrated in the 1970's. CT imaging was adopted as the “gold standard” for radiation therapy imaging early on due to its intrinsic spatial integrity, which comes from the physical process of X-ray attenuation. Despite the possibility of spatial distortions occurring in MRI, it is still very attractive as an imaging modality for radiotherapy as it has a much better soft tissue contrast than CT imaging and the ability to image physiological and metabolic information such as chemical tumor signals or oxygenation levels. The MRI artifacts that influence the spatial integrity of the data are related to undesired fluctuations in the magnetic field homogeneity and may be separated into two categories: 1) artifacts due to the scanner such as field inhomogeneities intrinsic to the magnet design and induced eddy currents due to gradient switching; and 2) artifacts due to the imaging subject, i.e., the intrinsic magnetic susceptibility of the patient. Modern MRI units are carefully characterized and employ reconstruction algorithms that may effectively eliminate artifacts due to the scanner. At high magnetic field strength, in the range of 1.0-3.0 T, magnetic susceptibility of the patient may produce significant distortions (which are proportional to field strength) that may often be eliminated by first acquiring susceptibility imaging data. Recently, many academic centers have started to employ MRI for radiation therapy treatment planning. Rather than dealing with patient related artifacts at high field, many radiation therapy centers have employed low field MRI units with 0.2-0.3 T for radiation therapy treatment planning, as these units diminish patient-susceptibility spatial distortions to insignificant levels. For dealing with intra-fraction organ motion MRI is highly favorable due to the fact that it is fast enough to track patient motions in real-time, has an easily adjustable and orientable field of view, and does not deliver any additional ionizing radiation to the patient which may increase the incidence of secondary carcinogenesis. Breath controlled and spirometer-gated fast multi-slice CT has recently been employed in an attempt to assess or model intra-fraction breathing motion by many research groups. Fast, single-slice MRI has also been employed in the assessment of intra-fraction motions, and dynamic parallel MRI is able to perform volumetric intra-fraction motion imaging. MRI holds a definite advantage over CT for fast repetitive imaging due to the need for CT imaging to deliver increasing doses to the patient. Concerns over increased secondary carcinogenesis due to whole-body dose already exist for IMRT and become significantly worse with the addition of repeated CT imaging.
In the prior art, two research groups appear to have simultaneously been attempting to develop a MRI unit integrated with a linac. In 2001, a patent was filed by Green which teaches an integrated MRI and linac device. In 2003, a group from the University of Utrecht in the Netherlands presented their design for an integrated MRI and linac device and has since reported dosimetric computations to test the feasibility of their device. The significant difficulty with integrating a MRI unit with a linac as opposed to a CT imaging unit, is that the magnetic field of the MRI unit makes the linac inoperable. It is well known that a charged particle moving at a velocity, V, in the presence of a magnetic field, B, experiences a Lorentz force given by F=q( V× B). The Lorentz force caused by the MRI unit will not allow electrons to be accelerated by the linac as they cannot travel in a linear path, effectively shutting the linac off. The high radiofrequency (RF) emittance of the linac will also cause problems with the RF transceiver system of the MRI unit, corrupting the signals required for image reconstruction and possibly destroying delicate circuitry. The integration of a linac with a MRI unit is a monumental engineering effort and has not been enabled.
Intensity modulated radiation therapy (IMRT) is a type of external beam treatment that is able to conform radiation to the size, shape and location of a tumor. IMRT is a major improvement as compared to conventional radiation treatment. The radiotherapy delivery method of IMRT is known in the art of radiation therapy and is described in a book by Steve Webb entitled “Intensity-Modulated Radiation Therapy” (IOP Publishing, 2001, ISBN 0750306998). This work of Webb is incorporated by reference into the application in its entirety and hereafter referred to as “Webb 2001”. The effectiveness of conventional radiation therapy is limited by imperfect targeting of tumors and insufficient radiation dosing. Because of these limitations, conventional radiation may expose excessive amounts of healthy tissue to radiation, thus causing negative side-effects or complications. With IMRT, the optimal 3D dose distribution, as defined by criteria known in the art (Webb 2001), is delivered to the tumor and dose to surrounding healthy tissue is minimized.
In a typical IMRT treatment procedure, the patient undergoes treatment planning x-ray CT imaging simulation with the possible addition of MRI simulation or a position emission tomography (PET) study to obtain metabolic information for disease targeting. When scanning takes place, the patient is immobilized in a manner consistent with treatment so that the imaging is completed with the highest degree of accuracy. A radiation oncologist or other affiliated health care professional typically analyzes these images and determines the 3D regions that need to be treated and 3D regions that need to be spared, such as critical structures, e.g. the spinal cord and surrounding organs. Based on this analysis, an IMRT treatment plan is developed using large-scale optimization.
IMRT relies on two advanced technologies. The first is inverse treatment planning. Through sophisticated algorithms using high speed computers an acceptable treatment plan is determined using an optimization process which is intended to deliver a prescribed uniform dose to the tumor while minimizing excessive exposure to surrounding healthy tissue. During inverse planning a large number (e.g. several thousands) of pencil beams or beamlets which comprise the radiation beam are independently targeted to the tumor or other target structure with high accuracy. Through optimization algorithms the non-uniform intensity distributions of the individual beamlets are determined to attain certain specific clinical objectives.
The second technology comprising IMRT generally utilizes multi-leaf collimators (MLC). This technology delivers the treatment plan derived from the inverse treatment planning system. A separate optimization called leaf sequencing is used to convert the set of beamlet fluences to an equivalent set of leaf motion instructions or static apertures with associated fluences. The MLC is typically composed of computer-controlled tungsten leaves that shift to form specific patterns, blocking the radiation beams according to the intensity profile from the treatment plan. As an alternative to MLC delivery, an attenuating filter may also be designed to match the fluence of beamlets. The current invention contemplates the fact that MLC delivery is capable of adjusting a delivery rapidly to account for intra-fraction organ motions while an attenuating filter cannot be actively adjusted.
After the plan is generated and quality control checking has been completed, the patient is immobilized and positioned on the treatment couch attempting to reproduce the positioning performed for the initial x-ray CT or magnetic resonance imaging. Radiation is then delivered to the patient via the MLC instructions or attenuation filter. This process is then repeated for many work weeks until the prescribed cumulative dose is assumed to be delivered.
Magnetic resonance imaging (MRI) is an advanced diagnostic imaging procedure that creates detailed images of internal bodily structures without the use of ionizing radiation, which is used in x-ray or megavoltage x-ray CT imaging. The diagnostic imaging method of MRI is known in the arts of radiology and radiation therapy and is described in the books by E. M. Haacke, R. W. Brown, M. R. Thompson, R. Venkatesan entitled Magnetic Resonance Imaging: Physical Principles and Sequence Design (John Wiley & Sons, 1999, ISBN 0-471-35128-8) and by Z.-P. Liang and P. C. Lauterbur entitled Principles of Magnetic Resonance Imaging: A Signal Processing Perspective. (IEEE Press 2000, ISBN 0-7803-4723-4). These works of Haacke et al. and Liang and Lauterbur are incorporated by reference into the application in their entirety and hereafter referred to as “Haacke et al. 1999” and “Liang and Lauterbur 2001”, respectively. MRI is able to produce detailed images through the use of a powerful main magnet, magnetic field gradient system, radiofrequency (RF) transceiver system, and an image reconstruction computer system. Open Magnetic Resonance Imaging (Open MRI) is an advanced form of MRI diagnostic imaging that uses a main magnet geometry which does not completely enclose the patient during imaging. MRI is a very attractive imaging modality for radiotherapy as it has a much better soft tissue contrast than CT imaging and the ability to image physiological and metabolic information such as spectroscopic chemical tumor signals or oxygenation levels. Many tracer agents exist and are under development for MRI to improve soft tissue contrast (e.g. Gadopentate dimeglumine for kidney or bowel enhancement, or Gadoterate meglumine for general contrast). Novel contrast agents are currently under development that will allow for the metabolic detection of tumors similar to PET imaging by employing either hyperpolarized liquids containing carbon 13, nitrogen 15, or similar stable isotopic agents or paramagnetic niosomes. All of these diagnostic MRI techniques enhance the accurate targeting of disease and help assess response to treatment in radiation therapy.
CT scanning for IMRT treatment planning is performed using thin sections (2-3 mm), sometimes after intravenous injection of an iodine-containing contrast medium and filmed at soft tissue and bone window and level settings. It has the advantage of being more widely available, cheaper than magnetic resonance imaging (MRI) and it may be calibrated to yield electron density information for treatment planning. Some patients who cannot be examined by MRI (due to claustrophobia, cardiac pacemaker, aneurism clips, etc.) may be scanned by CT.
The problem of patient setup errors, physiological changes, and organ motions during radiotherapy is currently a topic of great interest and significance in the field of radiation oncology. It is well know that the accuracy of conformal radiation therapy is significantly limited by changes in patient mass, location, orientation, articulated geometric configuration, and inter-fraction and intra-fraction organ motions (e.g. during respiration), both during a single delivery of dose (intrafraction changes, e.g., organ motions such as rectal distension by gas, bladder filling with urine, or thoracic breathing motion) and between daily dose deliveries (interfraction changes, e.g., physiological changes such as weight gain and tumor growth or shrinkage, and patient geometry changes). With the exception of the subject invention, no single effective method is known to account for all of these deviations simultaneously during each and every actual dose delivery. Current state-of-the-art imaging technology allows taking 2D and 3D megavoltage and orthovoltage x-ray CT “snap-shots” of patients before and after radiation delivery or may take time resolved 2D radiographs which have no soft tissue contrast during radiation delivery.
Great advances have been made in conformal radiation therapy; however, their true efficacy is not realized without complete real-time imaging guidance and control provided by the present invention. By the term “real-time imaging” we mean repetitive imaging that may be acquired fast enough to capture and resolve any intra-fraction organ motions that occur and result in significant changes in patient geometry while the dose from the radiation beams are being delivered. The data obtained by real-time imaging allows for the determination of the actual dose deposition in the patient. This is achieved by applying known techniques of deformable registration and interpolation to sum the doses delivered to the moving tissues and targets. This data taken over the entire multi-week course of radiotherapy, while the radiation beams are striking the patient and delivering the dose, allows for the quantitative determination of 3D in vivo dosimetry. Hence, the present invention enables the only effective means of assessing and controlling or eliminating organ motion related dose delivery errors.